Abstract

One of NAM’s Rotliegend prospects is situated at c. 2800 m below a major wrench-fault zone. Large lateral impedance contrasts across the faults are believed to be one of the main causes for the poor seismic imaging at the Rotliegend objective. Three-dimensional pre-stack depth migration (PSDM) seismic processing using hard-layer model updating was applied to the data in 2000. The results showed a significant improvement compared to previous post-stack depth migration (post-SDM) results and the original post-time migration processing. Despite these improvements in data quality, ambiguity still remained over the exact seismic pick of the top Rotliegend objective at the prospect. There was a clear case to optimize further the data quality at the prospect because only the high case interpretation proved to be economically viable.

A review of the 2000 PSDM processing concluded that there was still considerable scope for further improvement of the data by fine-tuning the velocity model near the complex re-activated fault systemas well as by detailed pre-stack data analysis. It was decided to embark on another 3D PSDM iteration to improve the structural definition of the prospect by using the latest available velocity modelling tools within Shell. The selected technique is based on non-layer constrained RMO measurements, followed by a velocity model inversion. This approach allows full and flexible interaction with the pre-stack data, a pre-requisite for proper velocity modelling in complicated wrench-fault systems.

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